Cuterebra larvae are opportunistic parasites of dogs, cats, and ferrets. Infestation is caused by the rodent or rabbit bot fly, Cuterebra spp (order Diptera, family Cuterebridae). Flies are usually host- and site-specific relative to their life cycle. However, rabbit Cuterebra are less host-specific and are usually associated with dog and cat infestations. Rarely, cats and dogs may be infested with Hypoderma spp or Dermatobia hominis. Ferrets housed outside may be infested by Hypoderma or Cuterebra spp.
Adult Cuterebra flies are large and bee-like and do not feed or bite. Females deposit eggs around the openings of animal nests, burrows, along runways of the normal hosts, or on stones or vegetation in these areas. A female fly may deposit 5–15 eggs per site and >2,000 eggs in her lifetime. Animals become infested as they pass through contaminated areas; the eggs hatch in response to heat from a nearby host. In the target host, the larvae enter the body through the mouth or nares during grooming or, less commonly, through open wounds. After penetration, the larvae migrate to various species-specific subcutaneous locations on the body, where they develop and communicate with the air through a breathing pore. After ~30 days, the larvae exit the skin, fall to the soil, and pupate. The duration of pupation varies depending on the environmental factors and winter diapause.
Cuterebra lesions are most common in the summer and fall when the larvae enlarge and produce a fistulous swelling ~1 cm in diameter. Dogs, cats, and ferrets are abnormal hosts for this parasite; aberrant migrations can involve the head, brain, nasal passages, pharynx, and eyelids. In the skin, typical lesions are seen around the head, neck, and trunk. The hair is often matted, and a subcutaneous swelling is present beneath the lesions. Cats often groom the area aggressively. Pain at the site is variable and usually associated with secondary infections. Purulent material may exude from the lesion; the most common differential diagnosis is an abscess or foreign body.
Free-roaming cats are more likely than indoor cats to develop lesions. Clinical signs are often associated with the CNS and typically occur between July and September. Cats may have depression, lethargy, or seizures; upper respiratory infections; or abnormal body temperatures (either hyperthermia or hypothermia). One key historical finding in cats with neurologic signs caused by Cuterebra infestations is an acute episode of violent sneezing weeks to months before clinical presentation. Common neurologic findings include blindness, abnormal mentation, and signs of unilateral prosencephalic disease. Idiopathic vestibular signs in cats may be due to aberrant migration of the parasite.
Yorkshire terriers may be at increased risk and can have a marked systemic inflammatory response, DIC, or both.
Definitive diagnosis is made by finding and identifying a larva. In cats, CT scans may help identify larvae. Second instar larvae are 5–10 mm long and are gray to cream in color. Third instar larvae are dark, thick, heavily spined, and are the stage most commonly seen by veterinarians.
Suspect lesions should be explored by carefully enlarging and probing the breathing pore or fistula with mosquito forceps. It is not unusual for the parasite to retreat into the opened pore, making it difficult to grasp. Covering the breathing pore with white petroleum jelly for 10−15 minutes before grasping the parasite can make it easier to remove. The lesion should not be squeezed, because this may rupture the larva and lead to a chronic foreign body reaction and secondary infection. There are anecdotal reports of larval rupture causing anaphylaxis. If possible, the larva should be removed in one piece; recurrent abscesses at the site of previous Cuterebra infestation suggest residual infection or remaining pieces of larva.
The area should be thoroughly flushed with sterile saline, debrided (if necessary), and allowed to heal by granulation. Healing may be slow. Ivermectin has been described as a treatment for cats with CNS cuterebriasis. Diphenhydramine (4 mg/kg, IM) is administered 1–2 hours before ivermectin (400 mcg/kg, SC) and dexamethasone (0.1 mg/kg, IV). Ivermectin is not approved for use in cats.
Owner education on the parasite's life cycle and risk factors to pets can help avoid recurrent episodes.
Opportunistic Cuterebra infestation of outdoor animals is most commonly seen in late summer and early fall.
Fluctuant soft tissue swellings with a fistula are typical and should be explored for parasitic larvae.
The opening/breathing pore is enlarged and the parasite removed by forceps.
The wound is cleaned and flushed to remove debris and allowed to heal by granulation.